1. Field of the Invention
The present invention concerns a thread clamp for a carrier thread gripper in a (flexible) rapier loom.
2. Discussion of Background Information
A carrier gripper must clamp a weft thread with utmost reliability, without which the thread could shift in the gripper or even escape therefrom, but it must handle the thread very gently without impairing its structure. This requirement is of particular importance in processing filament yarns, especially multifilament glass fiber, and especially if filament threads without twist are to be handled. This type of thread is used for very specific applications, e.g. in the manufacture of electronic components, and one of the main requirements a thread of this type must meet is the absolute absence of broken filaments, as any broken fibril may cause severe defects in the finished product. Clamping a thread of this type in the thread carrier gripper thus must be effected in a reliable manner, but without breaking any fibrils in the clamping device. Furthermore the clamping conditions prevailing in the clamping mechanism are to remain constant over time and thus are to be influenced as little as possible by wear of the elements, which is quite considerable, caused by the preparation to which such threads are subject, affecting the elements contacted by the thread.
Several options to solve the problem mentioned are known. However, these options have not proven satisfactory.
Thus, from EP-0477139, a carrier gripper for rapier looms is known comprising a cover plate and two lateral walls, which on the outer side of the lateral wall facing the warp shed forms a clamp for gripping the weft thread. The clamp is formed by a small clamping bar mounted on a support member fixed onto one of the lateral walls of the gripper. The clamping zone comprises, in a preferred variant of realization, a pliable clamping tape covered by a cushion of elastic material. Clamping of the thread is effected between the clamping bar and the elastic cushion. This clamping device, which has proven superior to the conventional clamping arrangements using levers and springs—as known from the state of the art and commonly used for clamping un-problematic threads of natural fibers, etc.—has been found insufficient, however, for meeting the extreme requirements described above. In the clamping arrangement cited, wear problems arise with the elastic cushion, in such a manner that the clamping force progressively diminishes over time until it no longer ensures reliable clamping of the thread.
The solution shown in EP-0584429 B1, according to which the thread is clamped and is inserted between two surfaces enclosing the wedge shaped gap, which surfaces furthermore present a curved cross-section shape, does not yield the desired results. On the one hand, it is subject to the effects of progressive wear, which tends to cause variations in the clamping force exerted onto the thread, and on the other hand, it has been found in practical use that adapting the clamping device to the thickness and the type of thread being processed is most difficult and requires a degree of precision in alignment which in weaving operations often is not available. Also this clamping device, developed just for meeting the demand specified for the present invention, did not yield the desired results.